Designing Stoichiometric Materials with Eu³⁺ for Dense, Optically Addressable Quantum Memory

Quantum memory built with stoichiometric rare-earth materials opposes conventional doped systems. Rather than containing ppm levels of weakly emitting, randomly distributed rare-earth dopants, stoichiometric compounds drastically increase concentrations while improving homogeneity. This, in theory, produces optical spectra with narrow inhomogeneous linewidths that can resolve hyperfine splits with hours-long coherence times, creating high densities of optically addressable rare-earth qubits. To explore how chemical factors influence stoichiometric materials’ linewidth, we initially studied known compounds with large Eu³⁺ separation to avoid undesirable cation interactions. We grew single crystals of an environmentally stable metal-organic framework with a long optical lifetime and an oxide with coexisting polymorphs, isolating one polymorph for the first time. But we wanted to expand the limited list of compounds with large Eu³⁺separation. Therefore, starting with known structures, we built 32 chemically diverse compounds using high probability Eu³⁺ and mononuclidic ion substitutions to create candidates with large separations and without linewidth broadening from isotopes. To guide our subsequent exploratory synthesis, we narrowed the candidates down to five with density functional theory stability calculations, which have rarely been used for lanthanide compounds. At least two of the predicted stable candidates, a fluoride and a phosphate, are synthesizable. Inhomogeneous linewidth studies will reveal how crystal quality and defect chemistry influence accessibility to the desirable, optically addressable Eu³⁺ transitions.